1. Field of the Invention
This invention relates to foil bearings and more particularly to utilizing a hydrostatic backup support for the foil in the foil hydrostatic bearing instead of a mechanical spring.
2. Description of the Related Art
Foil hydrostatic bearings employ foils for contacting a journal at zero to low speed operation. On the other side of the foil there is a high spring rate, hydrostatic fluid film between the housing and the foil. In a conventional foil bearing there is a low spring rate mechanical spring that pushes on the foil, to center or position the journal. The compliance of the low spring rate mechanical spring supporting the foils circumferencially 360.degree. enables it to position the rotor relative to the housing to define a static position before operation is initiated. It also enables the bearing foil to conform to some extent to misaligned and thermally distorted shaft loads.
Typically a foil bearing is used in light, small machines that operate at high speed and are used to pump low density and low viscosity fluids such as air. High viscosity fluids produce a shear load to fail the foil attachment points. Foil bearings have a low load capacity due to the hydrodynamic characteristics of the fluid at relatively low pressure and fixed geometry (length and diameter). As machines become larger and the density of the fluids pumped increases, the side loads experienced by the bearings at off design conditions increases. If the loads exceed the capacity of the bearings the bearings will fail.
The low spring rate mechanical backup springs limit the amount of force that can be applied to the bearing before it fails.